There is interest in many field of application to move from a pure Hydraulic Actuation System (HAS) to an Electro Mechanical Actuation System (EMAS). Examples are present both in aeronautics, where the concept of a “More Electric Aircraft” is becoming more and more important, and in ground/undersea vehicles. In the aeronautic field this trend is justified by the potential reduction of weight compared to a HAS and by the versatility offered by the electric approach. However, EMAS do not reach the same power density levels of hydraulic systems. The energy efficiency of EMAS can also be limited by the screw mechanism, i.e. self-locking screw, when the application requires position hold (like in aircraft seats). Moreover, EMAS suffer of a common issue that is the jamming, in addition to wear in the incorporated gears as well as in the screw mechanism, which can lead to backslash. In EMAS, a gearbox is necessary in order to lower the actuating torque thus permitting the use of small electric motors. This gearbox can negatively affect the overall volume and weight of the system. Furthermore, the high reduction ratio can have detrimental effect on the dynamic behavior of the system since, the inertia will be over-perceived from the motor. For these reasons, in particular for the jamming issue, for flight controls so far the EMAs are used only for backup purpose; also, they should be equipped in such a way to be easily decoupled by the other actuator during jamming.
The EHA solution can be seen as a more convenient way to transit to the “More Electric Aircraft”. The use of compact EHAs can permit to combine the power to weight advantage of hydraulic systems with the ease of control and wiring advantages of the electric systems. This concept has been well received in the aerospace field, and several solutions for EHA are currently available in the market.
The pump is an element in any hydraulic system, as concerns energy efficiency, noise emissions, life and reliability. In EHA systems, the pump design can be fixed displacement, being the flow controlled by the electric motor speed with a design suitable for miniaturization and permitting higher shaft speed. From this regard, external gear pumps offer high potential, considering manufacture cost and simplicity.
Various embodiments of the inventions described herein present novel and unobvious ways to improve electro-hydraulic actuation systems, and also positive displacement pumps.